Electron discharge tube with fluorescent screen



Feb. 23, 1943. j L VAN 2,311,672

ELECTRON DISCHARGE TUBE WITH FLUORESCENT SCREEN Filed Aug. 6, 1937 I 2 Sheets-Sheet l 3 i7 3 15 t t I 21 iii 7 E m '11 6 19 a i I 5 g 13 1 v 12 4 8 I In ve 21%; JAMES 0 mm J. D. LE VAN 2,311,672

UBE WITH FLUORESCENT SCREEN Feb. 23, 1943.

ELECTRON DISCHARGE Filed Aug. 6, 1957 2 Sheets-Sheet 2 i 71 0677 for N Wm E 5 w w Patented Feb. 23, 1943 UNITED STATES PATENT OFFICE ELECTRON DISCHARGE TUBE WITH FLUORESCENT SCREEN James D. Le Van, Belmont, 'Mass., assignor to Raytheon Production Corporation, Newton, Mass, a corporation of Delaware Application August 6, 1937, Serial No. 157,737 3 Claims. (01. zen-27.5)

This invention relates to electron discharge tubes, and more particularly to such tubes which are provided with a fluorescent screen upon which a fluorescent design is produced as a tuning indicator or as an indicator of other conditions'of the circuit associated with the tube.

An object of this invention is to devise such a tube in which the fluorescent screen is normally illuminated over a considerable portion of its area, and such illumination is substantially uniform throughout.

Another object of this invention is to devise such a tube in which the flow of current between the cathode and the fluorescent screen electrode remains substantially constant throughout the life of the tube.

A further object of this invention is to provide' means for producing a predetermined negative space charge region around the cathode of such a tube, whereby the uniform illumination of the fluorescent screen and a predetermined limitation of current flow between said cathode and said screen is produced.

The foregoing and other objects of my invention will be best understood from the following description of exemplifications thereof, reference being had to the accompanying drawings wherein:

Fig. l is a view partly in vertical section of one embodiment of my invention;

Fig. 2 is a view similar to- Fig. l, but taken at right angles to the plane of Fig. 1;

Fig. 3 is a cross-section taken along line 3-3 of Fig. i

Fig. 4 is a view similar to Fig. l of another modification of my invention;

Fig. 5 is a cross-section taken along line 55 of Fig. 4; and

Fig. 6 is a diagrammatic representation of a circuit in which my novel tube may be used.

Fluorescent screen tubes at the present time are used for a number of purposes, one of which is as a tuning indicator for radio-receiving sets.

In such tubes a fluorescent screen is provided which is subjected to electronic bombardment over the major portion thereof so as to produce an extended fluorescence upon said electrode. Means are provided to produce a design in said fluorescence usually by casting an electron shadow on said fluorescent screen in order to provide a tuning indication.

Various difllculties have arisen in connection with tubes of this kind. It has been found that during the life of such a tube the emission from the cathode which might originally be at a relatively low value would increase to such a high value that the fluorescent electrode would draw an excessive amount of current which would overheat the electrode and cause it to give oil gas, thus terminating the useful life of the entire tube. For example, it has been found that tubes which normally should have a current flow or the order of four milliamperes between the oathode and the fluorescent electrode would increase during life to about twenty-five to thirty milliamperes. The increased emission from the cathode would also cause an increased flow or current to the design-producing or electron shadow-casting electrode. This electrode is usually maintained at a positive potential, and has a resistance in series with it. Increased current flow to said design-producing electrode would cause an undue drop in the voltage impressed upon said electrode, and thus the desired control of the design produced on the fluorescent surface would be lost. A still further difilculty was that the extended fluorescence upon the fluorescent electrode was not uniform, but would possess a mottled appearance. It is believed that this was due to the fact that it is difiicult to make a cathode which emits uniformly over its entire surface. Any non-uniformity or emission has heretofore resulted in the mottled appearance mentioned above.

In accordance with my invention I have eliminated each of the above difficulties by interposing between the cathode and the fluorescent screen electrode, an auxiliary grid which is maintained at a. fixed potential, preferably by being electrically connected to the cathode.

, In Fig. 1 there is illustrated such a tube which I have described above, consisting of an envelope i containing an electrode assembly. The envelope l is provided with the usual stem 2 carrying a press 3 which supports an indirectlyheated cathode d, a control grid 5, and an anode 6. The cathode i is supported between the two insulating spacers l and 8 between which likewise are mounted the control grid 5 and the anode 6. The cathode i is electrically connected to a lead-in wire 9 sealed through the press 3 for establishing an external electrical connection to said cathode. Two lead-in wires Iii are provided to furnish electrical connection to the heater within the indirectly-heated cathode The grid 5 is connected to a lead-in wire I l, and the anode 6 is electrically connected to and supported by two standards 12 and i3 likewise sealed in the press 3. One of the standards l2 and It may be sealed through the press 3 in order to serve as lead-in connections for the anode.

' The envelope I is provided with a dome-shaped portion I4 within which is mounted a truncated cone-shaped electrode ll coated onits inner surface with a fluorescent material 13. This fluorescent material, when subjected to electron bombardment, produces a visible illumination which produce the fluorescence upon the electrode. The upper end of the cathode 4 is preferably covered by a shield 20 supported on a U- shaped frame 2|, which is electrically connected to the electrode l and is likewise carried by the insulating spacer II. The electrode I3 is electrically connected to the standard l3 by means of the conducting strip 2|. The standard I! is likewise sealed through the press 3, thus serving as a lead-in conductor for the electrode ii.

In order to produce fluorescence upon the coating it of the electrode I I, a positive potential is impressed upon the electrode I! through the lead-in conductor I 9. This normally would produce fluorescence on the entire surface of the electrode It. In order to produce a design in said fluorescence for the purpose of producing the desired indication, a control electrode 22 is supported adjacent one side of hte cathode 4. This control electrode is preferably in the form of a flat strip, as indicated. The electrode 22 is supported from the anode standard l3, and in this way is electrically connected to the anode and is subjected to the same voltage to which the anode 6 is subjected. When the electron stream flows from the cathode to the fluorescent electrode I5, a part of this stream will be intercepted by the control electrode 22, and a shadow or darkened portion 23 will appear, as indicated in Fig. 3, upon the surface of the electrode l5. As the potential becomes more positive upon the control electrode 22, the width of the shadow 23 decreases, thus giving an indication of the voltage conditions upon the anode.

In order to produce the various effects which I have mentioned above, I surround the cathode 4 which extends into the space surrounded by the electrode IS with a grid 24. This grid is electrically connected to and supported by the standard ID. This standard is preferably electrically connected to the cathode 4 by means of a con ductor 25. Thus the grid 24 is maintained at the cathode potential. In order for the grid 24 to accomplish the desired effects, the wires thereof must be of. a relatively small cross-sectional diameter. I have found that if the wires of the grid have a diameter of the order of magnitude of about flve mils or less, they do not tend to cast any shadow upon the electrode I! while if the wires are considerably greater'than this size, they do tend to cast a shadow.

It should be noted that the cathode 4 is provided with an electron-emissive coating 23' which extends only within the region surrounded by the control electrode 5 and the anode 3. The cathode 4 is likewise provided with an electron-emissive coating 26 which extends only within the region surrounded by the grid 24. The envelope megohm, to the positive terminal of a source of direct current 30, the other terminal of which is connected to ground. The cathode 4 is likewise connected to ground through a conductor 3|, and in this way the requisite potential for producing a flow of current between the cathode 4 and the anode 3 is provided. Variation in this current.

flow produces a varying voltage drop in the resistance 29, which in turn produces a varying voltage impressed upon the anode 3. Since the anode 3 is connected to the control electrode 22, this variable voltage likewise appears at the control electrode 22. In order to produce a variation in the voltage of the control electrode 22, the control grid 8 is supplied with a suitable control voltage from a conductor 33. This conductor may be connected through a resistance 34 to a conductor 35 connected to the control grid 3. when my novel tube is tobe used as a tuning indicator. the conductor 33 preferably is in the output of an automatic volume control system, and therefore the voltage which is impressed upon the conductor 33 is a voltage which is proportional to the intensity of the signal voltage. Since this intensity depends to a considerable extent upon the tuning of the set, this voltage may serve as a source of tuning indication. As is usual in such automatic voltage control'systems,

the resistance 34 is connected through a condenser 32 to ground, and an additional conductor 38 is provided which supplies the automatic volume control voltage to the grids of the proper tubes in the set. The fluorescent electrode I! may be provided with a suitable positive potential by means of a conductor 31 likewise connected to the positive terminal of the direct current source 30. As previously described, the grid 24 around the upper end of the cathode 4 is directly and electrically connected to the cathode 4.

When the system which I have described is put in operation, a negative voltage is impressed upon the control electrode 5 through the conductor 33, which negative voltage is proportional to the intensity of the signal supplied to the circuit. As this voltage increases, the control electrode 5 becomes increasingly negative, and

the current to the anode 3 decreases, producing a decreased drop through the resistor 23, therefore making the anode I and the control electrode 22 increasingly positive. The positive potential upon the fluorescent electrode ll causes an electron stream to emanate from the cathode 26, and produce a fluorescence over substantially the entire surface of the electrode l3, except where such electron stream is intercepted by the control electrode 22. As the control electrode 22 becomes increasingly positive with increase in the intensity of the signal voltage, the shadow 23 indicated in Fig. 3 becomes narrower, thus giving to the operator of the set an indication of the intensity of this signal voltage. As previously indicated, this serves as a tuning indication.

In absence of the grid 24, the current to the fluorescent electrode l2 and to the control grid 22 would depend to a large extent upon the emissivity of portion 23 of the cathode 4. Thus as the emissivity of this portion increases during life, the current to the electrode l would increase unduly, producing the discharge heretofore recited. An increase in current to the electrode 22 would cause an increase in the drop through the resistance 28, making the electrode 22 less positive. This would result in a decided broadening of the shadow 23 which is an undesirable feature. By providing the grid 24 around the portion 26 of the cathode 4, a definite space charge potential is established at the grid 24 which is independent of the emission of the cathode. Therefore, due to this fact, the current flow to the electrodes 22 and I5 remains substantially constant even though the electron emission from the portion 26 may vary considerably during the life of the tube. The grid 24 also acts as a difiusion member so that variations in the emissivity of difierent portions of the coating 26 are counteracted and the electron stream which issues from the grid 24 possesses a remarkable degree of uniformity, thus producing a uniform illumination throughout the fluorescence on the electrode [5.

The manner in which the grid 24 is supported in Figs. 1 and 2 possesses certain disadvantages inasmuch as the rod which supports the grid 24 is ordinarily larger than five mils in diameter and thus tends to cast a shadow upon the screen l5. Therefore, I prefer to mount the grid around the upper end of the cathode in the manner as shown in Figs. 4 and 5. In these figures parts corresponding exactly to those in Figs. 1 and 2 are numbered with corresponding reference numerals. The grid 24' surrounding the upper end of the cathode 4 is supported directly on the cathode itself. The grid 24' is formed of a single spiral wire welded at its upper end 38 to the upper end of the cathode 4. The lower end of the spiral is held in place on the cathode 4 by a few turns 39 closely surrounding the cathode 4 below the portion 28. The grid 24 mounted in this way has no supporting rods which tend to cast a shadow upon the fluorescent electrode I5. I have found that tubes made in this manner produce a uniform illumination substantially throughout the entire fluorescent surface of the electrode l5. Furthermore, this construction produces various advantages heretofore described.

This invention is not limited to the particular details of construction as described above as many equivalents will suggest themselves to those skilled in the art. For example, the grid 24 or 24' instead of being directly connected to the cathode could be provided with a separate lead, and any desired voltage impressed thereon would preferably be a negative voltage, which when of the proper value would produce the results which I have described above. Various other modifications will suggest themselves. It is accordingly desired that the appended claims be given a broad interpretation commensurate with the scope of the invention within the art.

What is claimed is:

1. An electron discharge device comprising a sealed envelope containing a cathode, an extended target electrode having a surface which fluoresces when subjected to a flow of electrons from said cathode, and an extended grid interposed between said cathode and target, said grid comprising wires having a diameter of the order of magnitude of five mils or less.

2. An electron discharge device comprising an envelope containing a straight thermionic cathode for emitting electrons, an anode surrounding said cathode and having a fluorescent coating exposed to direct view for receiving electrons from said cathode to produce a luminous pattern on the fluorescent coating of -said cathode, a control electrode comprising a straight conductor parallel to said cathode and having a position between said cathode and said anode for determining the width of said luminous pattern, depending on the voltage applied to said control electrode, and a foraminous electrode surrounding said cathode and interposed between said cathode and said control electrode, said foraminous electrode being electrically connected to said cathode and having a plurality of openings of sufficient number and size to cause the electrons emitted from said cathode to diffuse through said openings in an electron beam of substantially uniform cross-sectional intensity.

3. An electron discharge device comprising an envelope containing a thermionic cathode having two separate electron-emitting. sections, a grid and an anode surrounding one of said electron-emitting sections, an anode surrounding the other of said cathode-emitting sections and coated with a fluorescent material exposed to direct view for receiving electrons from said other cathode-emitting section to produce a luminous pattern on said anode, a control electrode comprising a straight conductor positioned between said other emitting section of said cathode and said anode coated with fluorescent material, said control electrode being electrically connected to and supported by the anode surrounding said grid and said first electron-emitting section, and a foraminous electrode surrounding said cathode and interposed between said cathode and said control electrode, said foraminous electrode being electrically connected to said cathode and having a plurality of openings of sumcient number and size to cause the electrons emitted from said cathode to diffuse through said openings in an electron beam of substantially uniform cross-sectional intensity.

JAMES D. LE VAN. 

